Biological materials, such as proteins, eukaryotic cells, bacteria and viruses, are generally unstable when stored in media or other liquid solutions. For example, enveloped viruses such as live influenza virus manufactured from egg allantoid fluid loose one log of potency, defined as Tissue Culture Infectious Dose (TCID50), in less than two to three weeks when stored under refrigerated temperature, i.e. approximately 4° C. At room temperature conditions (approximately 25° C.) and at warmer temperatures such as 37° C., the virus looses the such potency in a matter of days to hours, respectively. Lyophilization processes, where aqueous formulas are frozen and then dried by sublimation, are commonly used to stabilize these biological materials. Removal of water and substitution of protectant molecules, such as carbohydrates, can increase stability by preventing chemical degradation, denaturation, and growth of microbial contaminants.
In lyophilization (freeze-drying), the biological material is commonly mixed as a solution or suspension with protective agents, frozen, and then dehydrated by sublimation and secondary drying. The low temperatures of freezing and drying by sublimation can slow the kinetics of degradation reactions. However, the low temperatures and low surface to volume ratios involved can require long drying time periods. Often significant structural damage results in conventional freeze drying processes due to the slow can involve denaturation, aggregation, and other untoward physical stresses stemming from the ice crystal structures that are formed during the ice nucleation and propagation steps. For this reason, biomaterials that possess a cell wall or lipid membrane pose a significant challenge to preserving the bioactivity of larger and more complex entities such as viruses, bacteria, and cells.
Additionally, even under optimal freeze drying conditions, damage can occur during the secondary drying step. A recent study has suggested freeze drying induced damage occurs primarily during the secondary dehydration step when the last remaining amount of water is removed (Webb, S. D. Effects of annealing lyophilized and spray-lyophilized formulations of recombinant human interferon-gamma. J Pharm Sci 2003 April;92(4):715–29). Therefore, there is sufficient evidence to show that lyophilization and secondary drying processes can force a protein or cell, for example, to undergo significant chemical and physical changes. Such changes can result in loss of activity of the protein due to concentration of salts, precipitation/crystallization, shear stress, pH extremes, and residual moisture remaining through the freeze-drying.
Protective agents are chemicals that are added to a formulation to protect cells and molecules during freezing and to enhance stability during storage. For example, stabilizers for live virus vaccines generally include high concentrations of sugars such as sucrose, mannitol, or sorbitol to improve virus stability during lyophilization and storage. However, with membrane viruses, and other membranous biologicals, the protective agents may not penetrate adequately to protect active molecules within the membrane volume. Therefore a significant challenge remains to develop an optimal drying process and formulation to achieve adequate stability for thermally labile biologics.
Some of the problems with lyophilization are overcome by certain dry foam preservation processes. In U.S. Pat. No. 5,766,520, Preservation by Foam Formation, to Bronshtein, for example, biological solutions or suspensions in a solvent are thickened by first drying under a moderate vacuum before application of a strong vacuum to cause frothy boiling of the remaining solvent to form a dry stable foam. Normally, such boiling is avoided in processing of biological materials due to the oxidation and denaturation that can occur on bubble surfaces. In addition, boiling, even under vacuum, requires input of heat, which can endanger the stability of the bioactive material. These problems are reduced in Bronshtein by including protective agents, such as carbohydrates and surfactants, in the solution or suspension. Dry foam preservation processes of this type have the advantage of faster drying due to convection of the liquid during boiling and the large surface area presented by the foam. Reconstitution of such a dry foam can be rapid due to the presence of the hydrophilic protective agents and the large foam surface area. The dry foam can be milled to a fine powder to further improve reconstitution times or for administration of the biological material by inhalation.
The dry foam preservation processes described above is limited in its flexibility to protect a variety of biological materials. For example, the process rules out a freezing step and subsequent sublimation of the ice as a means to remove water from the foam. In the case of highly thermolabile materials, a freezing step can provide stability over the course of dehydration. Because freezing is avoided in Bronshtein, the formulation must be thickened before foaming and drying so that large amounts of water are not lost, along with latent heat, to freeze the foam. The avoidance of freezing requires the process to be conducted at lower vacuum level (7–24 Torr) than in conventional freeze drying or spray freeze drying process cycles. Boiling in Bronshtein, requires input of significant, and possibly destabilizing, amounts of heat to provide the necessary eruption of foam.
The Bronshtein dry foam process is not particularly well adapted to preservation of biological materials having lipid membranes. For example, the process is not well adapted to preservation of membranous biologicals, such as liposomes, viruses or viable cells. Lipid membranes often prevent penetration of the protective agents into enclosed volumes or prevent adequate removal of water from the enclosed volume. Without adequate penetration of protective agents, enzymatic processes, such as proteolysis, and chemical processes, such as oxidation and free radical attacks, can destroy the activity or viability of the membranous biological material. Hypoosmotic fluids remaining within membrane enclosed volumes can promote instability of the biological material.
A need remains for methods to preserve biological materials, such as proteins and membranous materials in storage, particularly at temperatures above freezing. Methods to prepare dry foam preservation matrices through processes with optional freezing and optional boiling steps, are desirable to suit the sensitivities of particular biologic materials. Compositions that can protect such biologicals in storage would provide benefits in medicine and scientific research. The present invention provides these and other features that will become apparent upon review of the following.